The stratum corneum, the outermost layer of skin, prevents ingress of harmful substances into the skin and moisture loss from the skin and at the same time, has a function of keeping flexibility or smooth appearance of the skin by retaining water in itself. Lipids called horny intercellular lipids (which will hereinafter be called “horny ICL”) exist so as to fill the interstices between numerous corneocytes.
In this horny ICL, almost 50% of the lipids are ceramides, and there are also cholesterol, cholesterol esters, fatty acids and the like. It is generally known that a decrease of horny ICL, especially of ceramides, triggers detrimental conditions in the skin, such as rough skin, dry skin or aged skin. The stratum corneum having a deteriorated function can be improved by the external use of ceramide as a component for improving the function of the stratum corneum.
Nonetheless, it is difficult to maintain the stability of a ceramide in a formulation, because it tends to crystallize readily and has a high melting point. When a ceramide is used in combination with another oil component, more than one surfactant is required depending on the differences of emulsifying activities thereof and in addition, the amount of the surfactants to be added tends to increase.
In Japanese Patent Application Laid-Open No. Hei 8-109121, described is a process of using an oil agent having good compatibility with a ceramide for the purpose of forming a stable composition of a ceramide without using a surfactant. This process is, however, not adequate for imparting a variety of functions and does not result in an adequate good skin feeling composition, because the oil agent to be used in combination is limited to those having good compatibility with a ceramide.
An oil-in-water emulsified cosmetic composition, on the other hand, has water as a continuous phase, so that it provides a light feeling and in addition, is refreshing to the eyes and to the touch. When a mixture of a ceramide with the other oil component is emulsified, such an oil-in-water emulsified cosmetic composition tends to worsen the feeling upon use and the emulsion itself becomes less stable.